Add a basic power model to the cpu cooling device to implement the
power cooling device API.  The power model uses the current frequency,
current load and OPPs for the power calculations.  The cpus must have
registered their OPPs using the OPP library.

Cc: Zhang Rui <rui.zhang@intel.com>
Cc: Eduardo Valentin <edubezval@gmail.com>
Signed-off-by: NKapileshwar Singh <kapileshwar.singh@arm.com>
Signed-off-by: NPunit Agrawal <punit.agrawal@arm.com>
Signed-off-by: NJavi Merino <javi.merino@arm.com>
Signed-off-by: NEduardo Valentin <edubezval@gmail.com>